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| United States Patent |
6,183,898
|
|
Koschany
, et al.
|
February 6, 2001
|
Gas diffusion electrode for polymer electrolyte membrane fuel cells
Abstract
A particularly inexpensive, homogeneous and porous gas diffusion electrode
which comprises at least one electrically conductive, hydrophobic and
gas-permeable gas diffusion layer, comprising a mechanically stable
support material which is impregnated with at least one electrically
conductive material having a bulk conductivity of .gtoreq.10 mS/cm is
produced. The gas diffusion electrode can be coated with a catalytically
active layer. The electrodes of the invention are particularly suitable
for use in fuel cells and electrolysis cells.
| Inventors:
|
Koschany; Arthur (Pocking, DE);
Schwesinger; Thomas (Kirchroth, DE);
Lucas; Christian (Planegg, DE);
Frank; Georg (Tubingen, DE);
Deckers; Gregor (Frankfurt, DE);
Soczka-Guth; Thomas (Hofheim, DE);
Bonsel; Harald (Waldems, DE)
|
| Assignee:
|
Hoescht Research & Technology Deutschland GmbH & Co. KG (Frankfurt, DE)
|
| Appl. No.:
|
388597 |
| Filed:
|
September 2, 1999 |
Foreign Application Priority Data
| Nov 28, 1995[DE] | 195 44 323 |
| Current U.S. Class: |
429/42; 29/623.1; 204/283; 429/44; 502/101 |
| Intern'l Class: |
H01M 004/86 |
| Field of Search: |
429/44,42
502/101
204/283
29/623.1
|
References Cited
U.S. Patent Documents
| 4804592 | Feb., 1989 | Vanderborgh et al. | 429/33.
|
| 5399184 | Mar., 1995 | Harada | 29/623.
|
| 5521020 | May., 1996 | Dhar | 429/42.
|
| 5863673 | Jan., 1999 | Campbell et al. | 429/44.
|
| Foreign Patent Documents |
| 0176831 A2 | Apr., 1986 | EP.
| |
| 0298690 A1 | Jan., 1989 | EP.
| |
| 0560295 A1 | Sep., 1993 | EP.
| |
| 0577291 A1 | Jan., 1994 | EP.
| |
| 0687023 A1 | Dec., 1995 | EP.
| |
| 1542346 | Oct., 1968 | FR.
| |
| 2258007 | Aug., 1975 | FR.
| |
| 2000363 | Jan., 1979 | GB.
| |
Other References
Derwent Publication, Week 9437. Patent Abstracts of Japan JP6223835 dated
Aug. 12, 1994.
|
Primary Examiner: Bell; Bruce F.
Attorney, Agent or Firm: Frommer Lawrence & Haug
Parent Case Text
This application is a continuation of Ser. No. 09/077,276 filed Feb. 10,
1999, now abandoned.
Claims
What is claimed is:
1. A gas diffusion electrode comprising at least one electrically
conductive, hydrophobic and gas-permeable gas diffusion layer, wherein the
gas diffusion layer comprises a mechanically stable support material which
is impregnated with at least one electrically conductive material having a
bulk conductivity of .gtoreq.10 mS/cm.sup.2, the mechanically stable
support material has a weight per unit area of <150 g/m.sup.2 and the gas
diffusion electrode has an electrical surface resistance of .ltoreq.100
m.OMEGA./cm.sup.2.
2. A gas diffusion electrode as claimed in claim 1, wherein the gas
diffusion electrode comprises at least one gas diffusion layer in which
the mechanically stable support material is a nonwoven, woven fabric or
paper.
3. A gas diffusion electrode as claimed in claim 2, wherein the
mechanically stable support material comprises carbon fibers, glass fibers
or fibers comprising organic polymers.
4. A gas diffusion electrode as claimed in claim 1, wherein the support
material has an open porosity in the range from 20 to 99.9%.
5. A gas diffusion electrode as claimed in claim 1, wherein the
electrically conductive material comprises carbon and/or a metal.
6. A gas diffusion electrode as claimed in claim 1, which comprises from
one to four gas diffusion layers.
7. A gas diffusion electrode as claimed in claim 1, which comprises a
catalytically active layer.
8. A gas diffusion electrode as claimed in claim 7, wherein the
catalytically active layer comprises
a) at least one catalytically active material and
b) one or more ion-conducting polymers and/or
c) one or more hydrophobic materials.
9. A gas diffusion electrode as claimed in claim 8, wherein the
catalytically active material is at least one metal of transition group
VIII or an alloy of one or more metals of transition group VIII, including
a metal of transition group IV.
10. A gas diffusion electrode as claimed in claim 8, wherein the mass ratio
of catalytically active material:ion-conducting polymer is in the range
from 1:100 to 100:1.
11. A gas diffusion electrode as claimed in claim 8, wherein the
concentration of catalytically active material decreases perpendicular to
the catalytic layer with increasing distance from the support material and
the concentration of ion-conducting polymer increases.
12. A gas diffusion electrode as claimed in claim 1, which is mechanically
reinforced by an electrically conductive mesh.
13. A gas diffusion electrode as claimed in claim 12, wherein the
electrically conductive mesh is a metal mesh or comprises a metal-coated
polymer.
14. A process for coating a gas diffusion electrode on one surface with a
catalytically active layer, wherein a gas diffusion electrode as claimed
in claim 1 is used and the coating procedure comprises the following
steps:
a) intensively mixing at least one catalytically active material with one
or more dissolved or suspended ion-conducting polymers,
b) applying the suspension prepared in step a) to one surface of the gas
diffusion electrode,
c) drying the layer applied.
15. The process as claimed in claim 14, wherein part of the suspension
liquid is evaporated before application of the suspension prepared in step
a).
16. The process as claimed in claim 14, wherein the steps b) and c) are
repeated one or more times.
17. The process as claimed in claim 14, wherein suspensions having a
different concentration of material and ion-conducting polymer are used in
successive layers.
18. A membrane-electrode unit comprising an anode, a cathode and a polymer
electrolyte membrane arranged between anode and cathode, wherein at least
one of the electrodes is a gas diffusion electrode as claimed in claim 1.
19. A fuel cell or electrolysis cell comprising the gas diffusion electrode
as claimed in claim 1.
20. A process for producing a gas diffusion electrode comprising at least
one electrically conductive, hydrophobic and gas-permeable gas diffusion
layer, which process comprises the following steps:
a) preparing a suspension comprising an electrically conductive material
and at least one liquid,
b) preparing one or more suspensions or solutions from a binder material
and at least one liquid,
c) intensively mixing the suspension prepared in step a) with at least one
of the suspensions prepared in step b),
d) impregnating a mechanically stable support material with the mixture
prepared in step c),
e) drying the impregnated support material,
f) sintering the impregnated support material at a temperature of at least
200.degree. C.,
where the mechanically stable support material has a weight per unit area
of <150 g/m.sup.2 and the gas diffusion electrode has an electrical
surface resistance of .ltoreq.100 m.OMEGA./cm.sup.2.
21. The process for producing a gas diffusion electrode as claimed in claim
20, wherein the electrically conductive material has an electrical bulk
conductivity of .gtoreq.10 mS/cm.sup.2.
22. The process for producing a gas diffusion electrode as claimed in claim
20, wherein the gas diffusion electrode comprises at least one gas
diffusion layer in which the mechanically stable support material is a
nonwoven, woven fabric or paper.
23. The process for producing a gas diffusion electrode as claimed in claim
20, wherein the step d) and e) are repeated one or more times.
24. The process for producing a gas diffusion electrode as claimed in claim
20, wherein one or more sintered gas diffusion layers are pressed together
at a pressure of up to 500 bar and a temperature of up to 400.degree. C.
25. The process for producing a gas diffusion electrode as claimed in claim
20, wherein the suspension prepared in step a) comprises a material for
reducing the surface tension.
26. The process for producing a gas diffusion electrode as claimed in claim
20, wherein the binder material and the electrically conductive material
are used in a mass ratio of from 1:100 to 100:1.
Description
The invention relates to a gas diffusion electrode and also a process for
its production, a process for coating the gas diffusion electrode with a
catalytically active layer and its use for fuel cells and electrolysis
cells.
In polymer electrolyte membrane fuel cells, a gas diffusion electrode is
used as electrode between polymer electrolyte membrane and current
collectors, e.g. bipolar plates. It has the function of conducting away
the current generated in the membrane and has to allow the reaction gases
to diffuse through to the catalytic layer. In addition, the gas diffusion
electrode should be water-repellent at least in the layer facing the
membrane in order to prevent water formed in the reaction from flooding
the pores of the gas diffusion electrode and thus blocking gas transport
to the catalytically active layer. For many applications, for example in
space travel and for use in automobiles, it is also important that the
materials used for constructing the cell stack are light and take up
little space but nevertheless have a high mechanical stability. Very
inexpensive production of the materials is always of interest.
For such gas diffusion electrodes, use has hitherto typically been made of
materials comprising graphitized fabric or graphitized papers which are
produced via an expensive thermal treatment (up to over 200.degree. C.)
(E-Tek, Inc. 1995 Catalogue, E-Tek, Inc. Natick. Mass. 01760, USA). The
gas diffusion electrodes comprising graphitized fabric often do not allow
oxygen, particularly atmospheric oxygen under low pressure, to diffuse
sufficiently well and are also relatively heavy. The dense structure is
necessary to obtain sufficient mechanical strength and a sufficiently high
conductivity of the fabric perpendicular to the fiber direction. Their
production requires high temperatures and an exact reaction procedure
which leads to a correspondingly high energy consumption and high prices.
The graphitized papers have the disadvantage that they are brittle and not
flexible and the pore structure of these papers is fixed and cannot be
changed without influencing the conductivity.
Also known are gas diffusion electrodes which comprise a hydrophobic,
porous support material which is sufficiently electrically conductive for
fuel cells, an intermediate layer which is not catalytically active and
comprises an electron conductor material, and a catalytically active layer
(EP-A-0 687 023). The intermediate layer which is not catalytically active
here comprises a mixture of an electron-conducting ionomer and a
proton-conducting ionomer. At a platinum loading of 0.21 mg/cm.sup.2, an
H.sub.2 pressure of 1.25 bar (absolute) and an air pressure of 1.8 bar
(absolute), a fuel cell using the gas diffusion electrodes described can
only achieve a maximum output of 200 mW/cm.sup.2 or an output of 163
mW/cm.sup.2 at a cell voltage of 0.6 V (Example 2, Table).
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a gas diffusion
electrode which is inexpensive to produce but mechanically stable, allows
oxygen, in particular oxygen from the air under a low superatmospheric
pressure, to diffuse readily, also has the necessary high electrical
conductivity and is mechanically stable and water-repellent.
It is an additional object of the invention to provide a process for
producing such a gas diffusion electrode.
It is also an object of the invention to provide a process for coating a
gas diffusion electrode with a catalytically active layer and to indicate
the use of the gas diffusion electrodes of the invention in fuel cells and
electrolysis cells.
These objects are achieved by the gas diffusion electrode as set forth
below.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows a polymer electrolyte membrane fuel cell according to the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The gas diffusion electrodes according to the invention are suitable for
fuel cells, in particular polymer electrolyte membrane fuel cells, and
polymer electrolyte membrane electrolysis cells. In polymer electrolyte
fuel cells, the gas diffusion electrodes of the invention can be used both
as an anode and as cathode. The gas diffusion electrodes of the invention
can be particularly advantageously used in polymer electrolyte membrane
fuel cells which use hydrogen as fuel and air as oxidant and are operated
at a low pressure of less than 0.5 bar above ambient pressure, preferably
less than 0.1 bar above ambient pressure.
The gas diffusion electrode of the invention comprises at least one gas
diffusion layer comprising a mechanically stable support material which is
impregnated with at least one electrically conductive material having a
bulk conductivity of .gtoreq.10 mS/cm.sup.2. In this context, the term
"impregnated" means that the pores (interstitial spaces between the
fibers) of the support material are essentially homogeneously filled with
the electrically conductive material.
In a preferred embodiment, the gas diffusion electrode of the invention
comprises from one to four gas diffusion layers.
The starting materials used for the gas diffusion electrodes of the
invention are very light, not necessarily electrically conductive but
mechanically stable support materials which comprise fibers, e.g. in the
form of nonwovens, papers or woven fabrics. The support material
preferably comprises carbon fibers, glass fibers or fibers comprising
organic polymers, for example polypropylene, polyester (polyethylene
terephthalate), polyphenylene sulfide or polyether ketones, to name but a
few. Particularly well suited materials are those having a weight per unit
area of <150 g/m.sup.2, preferably a weight per unit area in the range
from 10 to 100 g/m.sup.2. When using carbon materials as support
materials, nonwovens made of carbonized or graphitized fibers and having a
weight per unit area in the preferred range are particularly suitable. The
use of such materials gives two advantages: firstly, they are very light
and, secondly, they have a high open porosity. The open porosity of the
support materials which are preferably used is in the range from 20 to
99.9%, preferably from 40 to 99%, so that they can be very easily filled
with other materials and, as a result, the porosity, conductivity and the
hydrophobicity of the finished gas diffusion layer can be set in a
targeted manner by means of the filling materials, indeed over the entire
thickness of the gas diffusion layer.
To produce a gas diffusion electrode comprising at least one conductive,
hydrophobic and gas-permeable gas diffusion layer according to the
invention, a suspension is first prepared from an electrically conductive
material, preferably in powder form, which comprises, for example, carbon
(e.g. as carbon black) or else a metal which is insoluble or only very
slightly soluble in water and has a low oxidation sensitivity, e.g. Ti,
Au, Pt, Pd, Ag or Ni, and at least one liquid (e.g. water or low (C.sub.1
-C.sub.4) alcohols). The electrical bulk conductivity of the electrically
conductive materials used is, in particular, .gtoreq.10 mS/cm.sup.2,
preferably .gtoreq.100 mS/cm.sup.2. The particle size is, in particular,
in the range from 10 nm to 0.1 mm, preferably in the range from 50 nm to
0.05 mm. It can also prove to be advantageous to use mixtures of various
conductive powders or powders of alloys of conductive materials such as
stainless steel.
To reduce the surface tension, it is possible to add materials (additives
or detergents) such as lower alcohols. Such additives improve the ability
to prepare the suspension since they improve the wettability of the
electrically conductive material, e.g. the carbon black or the metal
powder, and thus make it more miscible with the suspension liquid. This
suspension, if desired also a plurality of such suspensions, is
intensively mixed with at least one suspension or solution of a binder
material, e.g. thermally stable polymers such as perfluorinated polymers
(fluorinated ethylene-propylene copolymers or polytetrafluoroethylene),
polyether ketones, polyether sulfones, polysulfones, polybenzimidazoles,
polyphenylene sulfides, polyimide, polyamide or polyphenylene oxides, in
at least one liquid, in particular water, N-methylpyrrolidone,
dimethylacetamide, dimethyl sulfoxide. The inherent viscosity of the
suspension (electrically conductive material, binder material and solvent)
is preferably in the range 5-0.01 dl/g, in particular in the range 2-0.05
dl/g.
Depending on the desired hydrophobicity of the gas diffusion layer, it is
also possible to use a plurality of the binders in admixture, e.g.
additional use of perfluorinated polymers in combination with a
non-fluorinated binder. The binder materials and the electrically
conductive material are preferably used in a mass ratio of from 1:100 to
100:1, particularly preferably in the range from 1:50 to 50:1.
The abovementioned support materials are thoroughly soaked with the
suspension mixture or the mixture is uniformly applied to the support
material so that the support material is essentially homogeneously
impregnated. The green gas diffusion layer produced in this way is
subsequently dried; the temperatures required for drying depend on the
type of liquids used and the support and binder materials used. In
general, drying at temperatures above room temperature is advantageous,
e.g. at temperatures above 80.degree. C., with drying being able to be
carried out either in air or under inert gas. The impregnation and drying
of the support material can be repeated one or more times. The support
material which has been impregnated in this way is subsequently sintered
at a temperature of at least 200.degree. C. in order to obtain an intimate
bond between the support material and the electrically conductive
material, but also within the conductive material itself. Sintering can
likewise be carried out in air or under inert gas. Depending on the
stability of the materials used, preference is given to sintering at
temperatures above 300.degree. C. The ratio of the weight per unit area of
the finished gas diffusion layer to the support material used is in the
range from 1.05 to 50, preferably in the range from 1.2 to 20.
The gas diffusion layer obtained in this way is particularly homogeneous,
porous and nevertheless mechanically very stable. This is achieved by
separating the mechanical stability function provided by the support
materials from the conductivity function provided by impregnation with the
conductive materials. Owing to the adjustable porosity, the gas diffusion
layer inhibits the diffusion of the gases required, in particular the
oxygen from the air, to a lesser extent than do the customarily
graphitized fabrics or papers. Owing to the intimate bond of the support
material to the conductive material achieved by means of the sintering
step, the conductivity of the gas diffusion layer of the invention is also
comparable with that of graphitized fabrics or papers and is sufficient
for use in fuel cells or electrolysis cells. Addition of a
hydrophobicizing agent (e.g. fluorinated polymers such as
polytetrafluoroethylene or fluorinated ethylene-propylene copolymers) to
the suspension comprising the conductive material enables a very uniform
hydrophobicization to be achieved over the cross section of the gas
diffusion layer. This leads to improved transport of the product water in
a fuel cell from the gas diffusion layer and thus out from the gas
diffusion electrode, and therefore leads to a further improvement in the
gas transport, in particular for the oxygen from the air.
To produce the finished gas diffusion electrode, one or more layers,
preferably from one to four layers, of the gas diffusion layers described
can be used. If more than one layer is used, it is advantageous to bond
these layers intimately to one another by means of a pressing or
lamination step, preferably at elevated temperature.
The gas diffusion electrode produced as described above can then be used,
for example, in a polymer electrolyte membrane fuel cell. Since the
above-described electrode does not contain a catalytically active layer,
it can be used in combination with a catalyst-coated membrane. As an
alternative, however, the gas diffusion electrode of the invention can
also be coated with a catalytically active layer.
The catalytic layer according to the invention has to be gas-permeable,
have electrical conductivity and H.sup.+ ion conductivity and, of course,
catalyze the desired reaction. These properties are obtained according to
the invention when the catalytically active layer is made very thin,
preferably having a thickness of from 1 to 100 .mu.m, preferably 3-50
.mu.m.
This layer comprises a.) at least one catalytically active material, b.)
one or more ion-conductive polymers, preferably selected from the group
consisting of sulfonated polyaromatics (e.g. polyether ketones, polyether
sulfones or polyphenylene sulfides), polybenzimidazoles and sulfonated,
perfluorinated polymers such as Nafion.RTM. (DuPont) or Flemion.RTM.
(Asahi Glass), and, if desired, c.) one or more hydrophobic materials,
e.g. fluorinated polymers such as polytetrafluoroethylene, polyfluorinated
ethylene-propylene copolymers or partially fluorinated polymers such as
polytrifluorostyrene. The ion-conducting polymers can be processed in the
form of suspensions or solutions in suitable solvents.
As catalytically active material, preference is given to noble metal
catalysts; in particular, the catalytically active material comprises at
least one metal of transition group VIII, e.g. platinum. Further preferred
materials are alloys of one or more metals of transition group VIII, in
particular comprising elements of transition group IV, where the content
of the metal of transition group VIII, e.g. Pt, in the alloy is in the
range from 20 to 95%, preferably from 30 to 90%.
The catalytically active materials (catalyst) can be supported or
unsupported. If a supported catalyst is used, the noble metal loading on
the support material is above 1% by weight, preferably above 2% by weight
A very favorable noble metal loading in the catalytically active layer is
less than 1 mg/cm.sup.2, preferably below 0.5 mg/cm.sup.2 and particularly
preferably below 0.3 mg/cm.sup.2 of the gas diffusion electrode. The mass
ratio of catalytically active material:ion-conducting polymer is typically
in the range from 1:100 to 100:1, preferably in the range from 1:10 to
20:1.
When using supported catalysts, preference is given to using carbon as
support material. The carbon support of the catalyst is electrically
conductive and porous, so that sufficient conductivity and
gas-permeability of the catalytic layer is ensured. The proton-conducting
polymer simultaneously serves as binder for the layer. The low layer
thickness according to the invention guarantees short transport paths and
thus low transport resistances for all materials required: electrons,
H.sup.+ ions and gas.
According to the invention, a gas diffusion electrode is coated as follows
on one surface with a catalytically active layer: a catalytically active
material, e.g. 20% of Pt on 80% of carbon (carbon black), is intensively
mixed with one or more dissolved or suspended ion-conducting polymers
(ionomers). The ion-conducting polymers which can be used have already
been described above by way of example. Suitable suspension media are
particularly water and alcohols, in particular C.sub.1 -C.sub.5 -alcohols,
or mixtures thereof. The suspension which comprises ionomer and catalyst
can, if desired, be diluted with a suitable liquid, e.g. water. The
suspension comprising the catalyst and the ionomer is applied to a
sheet-like side of the gas diffusion electrode, e.g. by spraying, printing
or brushing, and the layer which has been applied is then dried. It is
usually advantageous, before application of the suspension, to evaporate
part of the suspension medium mixture, e.g. part of the alcohols,
advantageously at slightly elevated temperature. This step enables the
surface tension of the suspension to be set such that the catalyst and
ionomer components present in the suspension wet essentially only the
surface of the gas diffusion layer, but do not penetrate into the interior
of this gas diffusion layer. In order to further minimize the inward
diffusion of the catalytically active layer, the gas diffusion layer can
also be impregnated beforehand with a liquid, e.g. water or alcohol, so
that the pores are then filled and penetration of the solution is
prevented.
The layer which has been applied in this way is subsequently dried. The
drying step of the catalytically active layer applied is usually carried
out at temperatures of from 10.degree. C. to 250.degree. C., preferably
from 15.degree. C. to 200.degree. C. and particularly preferably from
15.degree. C. to 150.degree. C. Drying can be carried out in air, but it
is also possible to use other drying media, e.g. nitrogen or noble gases.
Particularly good adhesion of the catalytically active layer is obtained
when the steps of application and drying are repeated one or more times.
The catalytically active layer does not necessarily have to have a
homogeneous thickness over its entire area on the gas diffusion layer;
rather, it is sometimes even advantageous if the thickness of the layer is
not the same everywhere, since this can reduce the roughness of the total
electrode. The catalytically active layer does not necessarily have to
have a homogeneous composition over its entire thickness; rather, it is
usually more favorable if there is a concentration gradient of
electrically conductive and ion-conductive material perpendicular to the
successive layers. Particularly when the catalytically active layer is
applied in a plurality of steps, as described above, selection of
different, suitable concentrations of catalytically active material and
ion-conducting polymer in the suspension makes it readily possible to
obtain layers whose concentration of catalytically active material
decreases perpendicular to the catalytic layer with increasing distance
from the support material and whose concentration of ion-conducting
polymer increases, i.e. those at the interface with the gas diffusion
layer are rich in catalyst and electron conductors but those on the free
surface of the electrode which is later coupled to the proton-conducting
membrane are rich in ionomer, which optimizes coupling of the electrode to
the membrane.
Such a distribution of electron conductor, catalyst and ion-conducting
polymer is also advantageous in that it is matched to the necessary
different concentrations of electrons and ions in the catalytically active
layer. Looking at the anode, for example, the fuel gas passing from the
gas diffusion layer into the catalytically active layer will be ionized to
an increasing extent over the catalyst on its way through the
catalytically active layer in the direction of the polymer electrolyte
membrane, so that the concentration of ions and thus the need for
ion-conducting material in the regions of the catalytically active layer
dose to the membrane is higher than in the regions adjoining the carbon
fiber nonwoven. On the other hand, the concentration of electrons and thus
the need for electron conductors is lower in the regions close to the
membrane, since the total number of electrons liberated does not pass
through these regions, but only the electrons liberated in the ionization
of the neutral tailgas still present in the respective region.
Analogously, in the catalytically active layer of the cathode, the
oxidation gas is ionized to an increasing extent by uptake of electrons on
its way through the catalytically active layer, so that, here too, in
regions close to the membrane the concentration of ions is higher and the
concentration of electrons is lower than in regions away from the
membrane.
The process of the invention for coating with the catalytically active
layer can be employed for any uncatalyzed gas diffusion electrode, in
particular for the gas diffusion electrode of the invention.
The gas diffusion electrode of the invention can be mechanically reinforced
by means of an electrically conductive mesh on the side opposite to that
with the catalytically active layer. Suitable meshes are metal meshes, but
also metal-coated meshes made of polymers such as polyesters, polyether
ketones, polyether sulfones, polysulfones or other polymers which have
continuous use temperatures above 100.degree. C. Metals suitable for the
meshes or the coating are noble metals such as Pt, Au, Ag, Ni or stainless
steels or carbon. The metal meshes can also be made of lower priced
materials such as steel if use is made of a protective coating of noble
metals or nickel. Particularly suitable meshes for the purposes of the
invention are square-mesh woven meshes having a mesh opening of from 0.4
to 0.8 mm and a wire thickness of from 0.12 to 0.28 mm, preferably of
nickel. Nickel is a favorable material in that it is chemically inert
under the conditions in the fuel cell and has a sufficiently low junction
resistance to the gas diffusion electrode. When assembling the fuel cell,
the mesh is installed on the side of the gas diffusion electrode away from
the membrane. Its functions are to ensure sufficient offtake of current
with a low junction resistance to the gas diffusion electrode, to
distribute the gases sufficiently uniformly over the area of the gas
diffusion electrode and at the same time to press the electrode uniformly
against the membrane.
If necessary, one or more gas diffusion layers can be combined into one gas
diffusion electrode. The use of more than one of the above-described gas
diffusion layers superposed on one another reduces, for example, the
danger of the mesh and/or parts of the current collectors, e.g. the
bipolar plates, pressing through to the membrane and damaging the latter.
Typically, a total of two or three impregnated gas diffusion layers per
electrode side are combined with one another. The use of more than four
superposed gas diffusion layers can lead to the gas diffusion no longer
being sufficient, which results in a decrease in the power output of the
fuel cell. To achieve good adhesion of the gas diffusion layers to one
another, the desired number of gas diffusion layers can be pressed
together, employing pressures of up to 500 bar and temperatures of up to
400.degree. C. Preferred conditions are pressures of up to 200 bar and
temperatures of up to 200.degree. C. The coating of one surface of such a
gas diffusion layer with catalyst is best carried out after forming the
intimate bond between the individual layers by means of pressing.
One or more of the gas diffusion electrodes of the invention can be
combined with a polymer electrolyte membrane to form a membrane-electrode
unit As polymer electrolyte membrane, it is possible to use any membranes.
Examples of these membranes are Nafion.RTM. (DuPont), Flemion.RTM., (Asahi
Glass), Gore-Select.RTM. (W. L. Gore & Assoc.), or membranes which are
described, for example, in the following publication: "New Materials for
Fuel Cell Systems 1", Proceedings of the 1st International Symposium on
new materials for fuel cell systems, Montreal, Quebec, Canada, Jul. 9-13,
1995, Les Editions de l'Ecole Polytechnique de Montreal, pp. 74-94. Of
particular interest are membranes without a fluorine content, since these
offer a series of advantages from an environmental point of view. For
optimum production of a membrane-electrode unit, the ionomer used for the
production of the catalytically active layer should, where possible, be a
type matched to the membrane: for coupling to a non-fluorinated membrane,
e.g. of sulfonated polyether ketone, the ionomer present in the
catalytically active layer should also be a sulfonated polyarylene. When
using a perfluorinated membrane, a perfluorinated ionomer is also used in
the active layer. However, other combinations of ionomer in the
catalytically active layer and in the membrane lead to satisfactory
membrane-electrode units. Depending on whether the gas diffusion electrode
bears a catalytically active layer or not, it is possible to use a
membrane without or with a catalytically active layer, where both parts
can naturally also bear a catalytic layer on their surfaces, so that the
bond is then established in the catalytic layer. To produce a
membrane-electrode unit, a gas diffusion electrode, which can be built up
of one or more impregnated gas diffusion layers, is placed on one side of
a polymer electrolyte membrane in its H.sup.+ form and subsequently
pressed on at pressures of up to 500 bar and temperatures of up to
250.degree. C. Preferred conditions are pressures of up to 300 bar and
temperatures of up to 200.degree. C. If the gas diffusion electrode
comprises the catalytically active layer, it is pressed onto the membrane
in such a way that the catalytically active layer is in contact with the
membrane. The contact between the electrodes on both sides of the membrane
and the membrane can be established in this way. The electrodes can, as a
matter of choice, be brought into contact with the membrane successively
or simultaneously.
To produce the membrane-electrode units, the catalytically active layers
between the gas diffusion layers and the membrane can be built up
identically or can have different compositions. When using pure hydrogen
(purity >99.9%), the catalyst content on the anode side can be selected so
as to be significantly lower than on the cathode side. The choice of
different catalytically active layers is of interest particularly when the
fuel cell operates using fuels other than pure hydrogen. It is then
advisable to use catalysts which, for example, have an increased
CO-tolerance, e.g. catalysts comprising alloys of Pt with Ru, on the
anode. In this case, too, it is appropriate to set different catalyst
contents for anode and cathode. The establishment of an intimate bond in
the abovementioned step significantly improves the electrical contact
between the catalytically active layer on the membrane and the gas
diffusion layer or between the catalytically active layer on the gas
diffusion layer and the membrane compared to simple clamping together, so
that the performance of the membrane-electrode unit in the fuel cell is
increased. Before installation of the membrane-electrode unit in a polymer
electrolyte membrane fuel cell, the gas diffusion electrodes can be
reinforced by fitting a mesh on the side facing away from the membrane.
The gas diffusion electrode of the invention has, compared to the known gas
diffusion electrodes, particularly low electrical surface resistances
which are in the range of .ltoreq.100 m.OMEGA./cm.sup.2, preferably
.ltoreq.60 m.OMEGA./cm.sup.2.
A particularly preferred embodiment of a fuel cell comprising a gas
diffusion electrode of the invention is shown in FIG. 1. Anode 1 and
cathode 1' are formed by the impregnated carbon fiber nonwovens 3 and 3'.
Anode 1 and cathode 1' each bear a catalyst layer 4 or 4' on their sides
facing the polymer electrolyte membrane 5. Anode 1 and cathode 1' together
with the polymer electrolyte membrane 5 form the membrane-electrode unit 6
or 6'. On their sides facing away from the membrane, anode 1 and cathode
1' are reinforced by conductive meshes 2 and 2' respectively. The bipolar
plates 7 and 7' form the outside of the cell on the anode and cathode
sides respectively.
Membrane-electrode units (MEUs) which comprise the gas diffusion electrodes
of the invention are suitable under all operating conditions for fuel
cells, i.e. they can be used with or without superatmospheric pressure, at
high and low gas flows and at temperatures up to 100.degree. C. Typical
power densities in operation using hydrogen and air are, depending on
operating conditions, up to 900 mW/cm.sup.2, in operation using hydrogen
and oxygen even up to 1.8 W/cm.sup.2.
Examples of the production and the properties of the gas diffusion
electrode of the invention:
EXAMPLE 1
45 g of carbon black (Vulcan XC 72.RTM.) are suspended in 450 ml of water
and 495 ml of isopropanol. This suspension is intensively mixed with 32.17
g of a polytetrafluoroethylene (PTFE) suspension (60% of Hostaflon.RTM.
fibers in aqueous suspension, manufactured by Hoechst AG, product number
TF5032). The resulting mixture is painted uniformly onto a carbonized
carbon fiber nonwoven (30 mg/m.sup.2) and the nonwoven is subsequently
dried at about 70.degree. C. The painting and drying are repeated twice.
After the last drying, the impregnated carbon fiber nonwoven is sintered
at 400.degree. C. for about 15 minutes. This gives a carbon fiber nonwoven
which is impregnated virtually uniformly over the total thickness and the
total area with Vulcan XC 72 and Hostaflon.
Coating of the gas diffusion electrode with a catalytically active layer
0.6 g of noble metal catalyst on a carbon support (20% Pt, 80% C) are
intensively mixed with 4.0 g of a 5% strength Nafion.RTM. solution (Nafion
dissolved in lower aliphatic alcohols and water) and 10.0 g of water. 2 g
of the alcohols present are then evaporated at 50.degree. C. in order to
increase the surface tension of the suspension. The suspension is then
sprayed onto an impregnated carbon fiber nonwoven and subsequently dried
at 80.degree. C. The spraying and drying is repeated twice. This results
in a gas diffusion electrode coated with catalyst and having a Pt loading
of about 0.2 mg/cm.sup.2.
Production of an MEU having a NAFION 115.RTM. membrane:
The membrane-electrode unit (MEU) having a Nafion 115.RTM. membrane in the
H.sup.+ form, which is, however, not preconditioned, is produced by
constructing a sandwich consisting of an above-described electrode, the
membrane and a further above-described electrode. The sandwich is then
pressed at a temperature of 130.degree. C. for 90 seconds at 250 bar and
an intimate bond is thus produced (MEU).
Results on the MEU in a fuel cell:
The performance of the MEU produced in this way was then studied in a test
cell. The fuel cell was operated under the following conditions: H.sub.2
gauge pressure 0.5 bar, not humidified, air gauge pressure about 60 mbar,
air index 16, the air is humidified. The temperature of the cell was
65.degree. C. Ni meshes are used as power conductors. After a running-in
period of the MEU, during which the membrane accumulates the amount of
water required for a high conductivity, the following performance data are
obtained:
Current density Power density
Voltage (mV) (mA/cm.sup.2) (mW/cm.sup.2)
1002 0 0
750 208 151
700 300 210
600 563 338
500 700 350
EXAMPLE 2
Production of a gas diffusion layer as in Example 1, but using a membrane
of sulfonated polyether ether ketone ketone (PEEKK) having a thickness of
40 .mu.m (measured in the dry state) and an ion-exchange equivalent of
1.46 mmol or H.sup.+ /g. After being produced, the membrane was boiled in
deionized water for 2 hours and subsequently dried again under ambient
conditions, so that the membrane was largely dry during installation. The
electrodes from Example 1 were laid on both sides of the membrane and
subsequently pressed at room temperature to form an MEU.
The MEU was installed in the test cell and operated under the following
test conditions: cell temperature 80.degree. C., H.sub.2 gauge pressure
<100 bar, humidification at 80.degree. C., flow 2, air gauge pressure <100
mbar, air index 5.5, humidification at 80.degree. C.
The following performance data were able to be achieved:
Current density Power density
Voltage (mV) (mA/cm.sup.2) (mW/cm.sup.2)
980 0 0
750 132 99
700 240 168
600 520 312
500 710 355
EXAMPLE 3
Comparative Example
4 layers of the gas diffusion layer produced as described in Example 1 or 2
are installed as a circular sheet having an area of 12 cm.sup.2 in a cell
block of a fuel cell. The gas diffusion layers were then supplied with a
current of 1 A/cm.sup.2 and the voltage drop across the cell block was
measured. The surface resistance of the gas diffusion layers including the
junction resistances to the cell block is 40 m.OMEGA./cm.sup.2 when the
parts are pressed together at a pressure of about 10 bar.
This experiment was repeated under identical conditions using the
unmodified carbon fiber nonwovens employed as starting material. The
resistance of the untreated carbon fiber nonwoven layers was 330
m.OMEGA./cm.sup.2 and thus about 8 times greater than the resistance of
the gas diffusion layers produced according to the invention.
EXAMPLE 4
Use of a glass fiber nonwoven having a weight per unit area of 30
g/m.sup.2, with the individual glass fibers having a diameter of 12 .mu.m,
as mechanical stabilization for the gas diffusion electrode. The surface
resistance of the glass fiber nonwoven is greater than 100
k.OMEGA./cm.sup.2. To produce an electrode, two gas diffusion layers with
glass fiber nonwoven were used. The production of the individual gas
diffusion layers is carried out by a method similar to Example 1, i.e. the
glass fiber nonwoven is impregnated as homogeneously as possible over its
total thickness with a suspension of carbon black/PTFE, dried and then
sintered. The formulation used for the suspension and the treatment steps
are similar to Example 1. The finished gas diffusion electrodes are then
provided on one side with a catalytically active layer, likewise similar
to Example 1. The platinum content of the catalytically active layer is
0.2 mg/cm.sup.2.
The surface resistance of the gas diffusion electrodes was 80
m.OMEGA./cm.sup.2, i.e. more than 10.sup.6 times smaller than the
resistance of the glass fibers alone!
To produce an MEU, two glass fiber electrodes produced in this way were
pressed together with a Gore Select.RTM. membrane (40 .mu.m) (from W. L.
Gore & Assoc.) at 90.degree. C. and 80 bar to form an MEU.
The performance of the MEU was then studied under the conditions of Example
1. This gave the following data:
Current density Power density
Voltage (mV) (mA/cm.sup.2) (mW/cm.sup.2)
980 0 0
750 263 196
700 371 260
650 500 325
550 750 412
EXAMPLE 5
Production of the MEU using a method similar to Example 2, but the
thickness of the membrane used is in this case only 25 .mu.m. This MEU was
studied under the following conditions:
Cell temperature: 50.degree. C., use of hydrogen and oxygen at 3 bar
absolute pressure and a flow of about 2. The humidifiers for H.sub.2 and
O.sub.2 were operated at ambient temperature, i.e. 22.degree. C., so that
the gases were only about 30%-saturated with water vapor. This gave the
following data:
Current density Power density
Voltage (mV) (mA/cm.sup.2) (mW/cm.sup.2)
980 0 0
750 374 281
700 580 406
600 1000 600
500 1395 697
Here, for example, the output found at 700 mV was able to be maintained for
a number of hours at the low humidification.
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